Radial tires of run flat type (hereinafter referred to as a run flat tire) are mainly used in vehicles wherein a load applied to the tire is relatively small such as passenger cars, light trucks, small size trucks and the like. In the run flat tire, it is required that when the tire is rendered into a flat state through puncture during not only the running on general-purpose roads but also the high-speed running on express ways, it is possible to safely and surely run the tire up to a site capable of conducting tire exchange over a given distance, for example, not less than 300 km without damaging the steering stability of the vehicle, particularly passenger car and causing the separation off from a rim or the breakage of the tire.
For this end, run flat tires having various structures are proposed sometimes in a combination with a fully worked-out rim to be used. These proposed tires are roughly divided into super low section profile tires having an aspect ratio of less than 60 and tires having a relatively high section height corresponding to an aspect ratio of not less than 60.
The run flat tires mostly and practically used in markets among the above super low section profile tires are disclosed, for example, in JP-B-45-40483, JP-B-50-12921, JP-A-49, 70303, JP-A-49-116702, JP-A-50-59902, JP-A-50-60905, JP-A-50-60906, JP-A-50-60907, JP-A-50-78003, JP-A-50-111704, JP-A-50-121902, JP-A-50-138502, JP-A-51-20301, JP-A-51-64203 and JP-A-51-69804.
Giving an example of the tires disclosed in the above publications, as shown in FIG. 7, a tire 20 has a structure that a pair of thick reinforcing rubber layers 9 having a crescent shape at section are arranged at an inner face side of an innermost carcass ply 6-1 in a radial carcass 6 extending from a bead portion 2 through a sidewall portion 3 to a tread portion 4. However, this type of the tire is unavoidable in a point that the cost becomes higher and is frequently mounted onto an expensive grade of a vehicle supposing the high-speed running such as sport car, sport-type car or the like.
In the tire 20 having the thick reinforcing rubber layer 9, in order to mitigate the degree of crushed deformation during the running at run flat state under loading (hereinafter referred to as run flat running or running under low internal pressure) as far as possible, the radial carcass 6 has a two or more ply structure comprising a turnup ply 6-1 wound around a bead core 5 from an inside of the tire toward an outside thereof and a down ply 6-2 enveloping the turnup ply 6-1 from an outside thereof, and a hard stiffener rubber 8 extending from an outer peripheral face of the bead core 5 near to a position of a tire maximum width is disposed between the turnup ply 6-1 and the down ply 6-2, and a rubberized Kevlar cord or steel cord layer (a layer called as an insert ply) is arranged so as to extend from the bead portion 2 to the sidewall portion 3, if necessary.
On the other hand, the tires having the relatively high section height are frequently used in not only high-grade imported passenger cars and high-grade domestic passenger cars having a relatively large displacement but also light trucks and small size trucks. This type of the tire is possible to run at run flat state in cooperation with a rim fitted thereto, wherein a type of incorporating a protection member (core) for pushing the bead portion toward a rim flange into the rim is the main current as disclosed in JP-U-56-143102, JP-Y-4-11842, JP-U-2-64405, JP-U-2-64406, JP-A-5-104915, JP-A-6-48125 and JP-A-6-270617.
In the tire having the structure shown in FIG. 7, it is proposed and practiced to increase the thickness or height of the stiffener rubber 8 and the reinforcing rubber layer 9, or largely increase the hardness and modulus of rubbers 8, 9 themselves for improving the durability in the run flat running. These improving means are proposed under an intention that region a in the vicinity of the bead portion 2 and region β of the sidewall portion 3 being trouble region in the run flat running as shown in FIG. 8 are reinforced and at the same time the reinforcing balance of both regions α, β is optimized to reduce strain at the region a and strain at the region β together.
However, as the reinforcing degree of each of the stiffener rubber 8 and the reinforcing rubber layer 9 increases, the occurrence of troubles in the region a and the occurrence of troubles in the region β as a tire trouble in the run flat running can be avoided, while the trouble site merely and rapidly shifts to a region γ of a shoulder portion including an end portion of a belt 7 and an end portion of the reinforcing rubber layer 9 as shown in FIG. 7. Finally, the thus improved conventional run flat tire can not attain the run flat runnable distance of not less than 300 km and support the high-speed running on the expressway as user's demand. Therefore, it is still desired to develop tires having a run flat durability capable of running over not less than 300 km and supporting the high-speed running on the expressway.
And also, the means of incorporating the core into the rim has a problem firstly in the rim assembling property because the assembling of the tire onto a wheel is not easy. Further, the big rise of weight in the tire and in the tire-wheel assembly is unavoidable and also unspring weight of the vehicle is largely increased to considerably damage the ride comfort of the vehicle against vibrations. That is, there is a problem that these inconveniences become unsuitable in not only the high grade passenger cars but also the light tuck and small-size truck.
It is, therefore, an object of the invention to provide a pneumatic tire particularly having an aspect ratio of not less than 60 which can hold a good rim assembling property without causing weight increase in tire-rim assembly and remarkable cost rise, guarantee the safe high-speed running of a vehicle such as passenger car, light truck, small size truck or the like even if rapid air leakage is caused by puncture or the like, and develop the performance of preventing separation off of the tire from the rim during the run flat running over not less than 300 km and the durability performance.
It is another object of the invention to provide a pneumatic tire having an aspect ratio of not less than 60 and an excellent ride comfort against vibrations during the run flat running, particularly a run flat tire for passenger car.